Gene Steuerle once again makes more sense than most other people, either on the right or the left, trotting out a proposal he and I made years ago:

Clinton may be suggesting more of a commandment than she can enforce, Obama less of an inducement than he can provide … Let me offer my solution (again!) for one enforceable way to extend "mandates" … [T]he federal government could make the $1,000 per child credit, which most taxpayers receive, available only for those who buy insurance for their children. To extend the mandate further down the income scale, I would beef up and extend the credit to those not eligible. This method of denying public benefits is more easily enforced than trying to run around and knock on doors to collect sizable sums of money.

(See Gene Steuerle, Clinton Versus Obama on Health Mandates.)